1. Field of the Invention
The present invention relates to an ultrasound diagnostic apparatus, and more particularly to an ultrasound diagnostic apparatus for displaying spectrum data of a Doppler signal obtained by transmitting/ receiving an ultrasound wave to/from an object and performing various measurements based on the spectrum data.
2. Description of the Related Art
The ultrasound diagnostic apparatus emits an ultrasound pulse or a continuous wave generated from a piezoelectric transducer incorporated in an ultrasound probe into the subject(patient), and receiving an ultrasound reflected wave generated by a difference in acoustic impedance between subject tissues and displaying it on a monitor. This diagnostic method is classified into ultrasound tomography for displaying a two-dimensional image by using an ultrasound pulse, and a so-called ultrasound Doppler spectrum method for displaying a time dependent change of a Doppler spectrum (referred to as spectrum data, hereinafter) obtained by frequency-analyzing a Doppler deviation component generated when an ultrasound pulse or an ultrasound continuous wave is applied to a moving reflector (e.g., blood or tissue) in the subject.
The ultrasound tomography includes a B-mode method for displaying a two-dimensional distribution of reflection intensity, and a color Doppler method for two-dimensionally displaying speed information of a blood flow or a tissue in color by using a Doppler component. These methods have widely been used for diagnosing patterns and functions of various organs since a two-dimensional image can be easily observed in real time by a simple operation of only bringing the ultrasound probe into contact with a body surface.
The ultrasound Doppler spectrum method includes a method for using a pulse wave (pulse Doppler method) and a method for using a continuous wave (continuous wave Doppler method) as in the case of the ultrasound tomography, both of which are used for quantitatively measuring a moving speed of a blood flow or a tissue. Especially, the pulse Doppler method is applied to a case in which resolution of a distance direction (ultrasound transmitting/receiving direction) is necessary and a flow velocity or a moving speed is relatively slow, and the continuous wave Doppler method is applied to a case in which a blood flow velocity is extremely fast and a Doppler spectrum is folded many times as in the case of a cardiac valve defect patient.
The spectrum data in the ultrasound spectrum method is usually displayed in a sonogram form in which an abscissa of a display screen corresponds to a time axis, an ordinate to a frequency component, and luminance to a size of power of each frequency component. For severity determination of the cardiac valve defect patient or the like, a maximum frequency component value (maximum flow velocity value) of a reverse-flow component spectrum displayed in the sonogram form or a tracing waveform generated based on the maximum frequency component of this spectrum is generally used.
FIGS. 16A to 16C show a conventional spectrum data generating method: FIG. 16A showing a Doppler spectrum 151 obtained by subjecting an ultrasound Doppler signal obtained from a subject by the pulse Doppler method or the continuous wave Doppler method to high-speed Fourier transformation (FFT) analysis, in which an ordinate corresponds to a Doppler deviation frequency and an abscissa corresponds to a size (power) of a spectrum, FIG. 16B spectrum data 152 indicating a time dependent change of the Doppler spectrum 151, in which an ordinate corresponds to a Doppler deviation frequency, an abscissa corresponds to an observation time, power of the Doppler spectrum 151 is represented by luminance, and an electrocardiographic wave (ECG wave) 153 collected with the spectrum data 152 is simultaneously displayed, and FIG. 16C a time dependent change of a size of an ultrasound wave (transmitted acoustic power, hereinafter) emitted from the piezoelectric transducer in the collection of the spectrum data. Conventionally, always constant transmitted acoustic power has been used as shown in FIG. 16C.
Meanwhile, it has conventionally been known that when an ultrasound wave emitted to the subject is reflected on the moving reflector, random interferences occur between reflected waves, resulting in interference noises (speckle noises) in the Doppler spectrum. That is, as shown in FIG. 16A, irregularities occur in the calculated Doppler spectrum 151 (solid line). Accordingly, discontinuous patterns also occur in the spectrum data 152 of FIG. 16B showing the time dependent change of the Doppler spectrum 151, causing a difficulty of accurately observing a time dependent change of a maximum frequency component (maximum flow velocity value) or the like.
The influence of the interference noises is conspicuous when an S/N ratio of the Doppler spectrum is small because reflection intensity from the moving reflector is small. For example, in the case of determining the severity of the cardiac valve defect patient or the like by tracing 155 of a maximum negative frequency component in the spectrum data, not only accurate automatic or manual tracing is difficult but also much time is necessary for tracing in the case of manual tracing, creating problems such as an increase in the load of a tracing operator and the like.
To deal with the problems, there has been proposed a method of reducing the interference noises by averaging movements in an observation time direction by frequency component units of the spectrum data (e.g., pp. 4 to 6, FIGS. 1 and 2, Jpn. Pat. Appln. KOKAI Publication No. 6-327672).
According to the method described in the Jpn. Pat. Appln. KOKAI Publication No. 6-327672, the influence of the interference noises is reduced, and thus a peripheral portion in the spectrum data, i.e., a maximum frequency component or the like, can be continuously and smoothly displayed, improving visibility of tracing data. However, to obtain such an effect, the movement averaging process must be carried out for a relatively long observation time. As a result, clearness of the spectrum data is greatly reduced.
Furthermore, as a method of improving the S/N ratio of the ultrasound Doppler signal, a method of increasing the transmitted acoustic power of the ultrasound probe is available. However, there are limits imposed by heat generation regulations or acoustic power regulations established by Food and Drug Administration (FDA) or the like. Especially, an upper limit is imposed on the transmitted acoustic power by regulations regarding a surface temperature of the ultrasound probe or a temperature increase of a biomedical tissue (so-called thermal index). In a normal apparatus, spectrum data is generated by using transmitted acoustic power near an upper limit of a permissible value. Thus, it is impossible to further increase the transmitted acoustic power.